Phytoplankton blooms - accumulations of microscopic algae on the surface of oceans and freshwater bodies - can do both good and harm.

They form the bottom of the food web and so can fuel fisheries, as well as sucking up carbon dioxide, the planet-heating gas.

But some algae can also produce toxins that accumulate in the food web, with the potential to make people ill, kill fish or wipe out a fishery.

That is why they need measuring, say the authors of a new paper, published in peer-reviewed journal Nature, that has mapped blooms globally.

They assessed 760,000 satellite images from 2003 to 2020 and found the total area had increased 13.2%.

"Phytoplankton blooms can be really beneficial," said study co-author Don Anderson who leads the US National Office for Harmful Algal Blooms at Woods Hole Oceanographic Institution.

"Many of the really productive fisheries in the world's ocean are driven by blooms. It's important to understand why they might be changing."

Blooms were more prevalent in areas near river estuaries than along coastlines without major river discharge.

"Unfortunately, algal bloom frequency and distribution are projected to increase with future climate change," as warmer temperatures could stimulate more blooms, the authors said, damaging water ecosystems, fisheries and coastal resources.

Large quantities of phytoplankton can also starve the ocean of oxygen, leading to "dead zones" that wreak chaos on the food chain and fisheries.

A 2016 algal bloom near Chile cost salmon farms $800m (£670m).

The researchers hope their findings will help minimise the damage from blooms by forecasting them better, and improve policy to stopping nutrients from fertiliser running off the land.

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